(1) Field of the Invention
This invention relates to holography, and in particular to holograms and processes for forming holograms which are capable of reconstructing three-dimensional multicolor images.
(2) Description of the Prior Art
A major desirable aspect of holograms is that a true three-dimensional image may be reconstructed therefrom. Typically, focused image holograms are formed by directing a beam of coherent radiation such as from a laser through a beam splitter to form two beams, one of which is used as a reference beam and the other of which is directed onto an object. An object beam formed from the coherent light reflected from (or passed through) the object, is then directed at one angle onto a suitable recording medium, while the reference beam is directed onto the medium from another direction to interfere with the object beam and thereby form in the medium a diffraction pattern or hologram corresponding to the object. While such a hologram may be reconstructed to enable viewing the image in three dimensions, a well-resolved, reconstructed image generally tends toward monochromaticity.
A number of techniques have been proposed for enabling the reconstruction of a three-dimensional multicolor image. In one such technique, holographic diffraction patterns prepared by coherent light of three primary colors are superimposed, or multiplexed, onto a panchromatic medium. While such a technique enables a three-dimensional multicolor image to be seen, a number of undesirable ghost images are formed. To minimize cross-talk resulting in such ghost images, volume holograms containing superimposed diffraction patterns corresponding to each of the primary colors have been formed in a thick emulsion. See, for example, that set forth in U.S. Pat. No. 3,532,406 (Hartman). Such volume holograms have been noted to possess several disadvantages:
(1) the thick emulsions generally shrink during development and thus changes the spacing between the fringes, and hence of the reproduced colors; and
(2) the presence of overlapping reference beams results in the presence of beat frequencies and ghost images.
Subsequent such developments, Friesem et al, "Multicolor Wavefront Reconstruction", Appl. Optics. Vol. 6, No. 3, pp. 529-536 (Mar. 1967), disclosed that such problems in volume holograms could be further minimized. In the work there reported, the angle between the reference and object beam was greater than 100.degree., the useful coherence length of the radiation was as long as possible and effects of vibrations, film transfer characteristics and film shrinkage were minimized.
In order to further avoid ghost images in multicolor holograms, multicolor holograms have been formed in two-dimensional planar media using a spatial multiplexing technique in which the interference fringe patterns corresponding to each color are recorded on different groups of small areas dispersed over the photographic medium. See U.S. Pat. No. 3,567,305 (Collier et al). In one technique there disclosed, an aperture mask consisting of thin stripes of alternating transparent and opaque areas is placed over a photographic medium and a first pair of beams including a reference beam and an object beam corresponding to one color of the object are directed onto the mask such that a first group of small areas or stripes are exposed. The position of the mask is changed and a second pair of beams corresponding to a second color are directed onto a previously unexposed second group of small areas or stripes. Alternatively, that patent indicates that a filter mask having a plurality of parallel stripes, any one of which is capable of transmitting only one of the primary colors, red, green or blue, may be placed in contact with the medium and light from three pairs of coherent beams corresponding to each color directed thereat. The stripes are said to be arranged so that each of the colors is distributed uniformly throughout the mask. On reconstruction, the colors of the illuminating light are then also controlled such that light of any given color illuminates only that portion of the hologram which had been formed with the same color, such as by placing the same mask used in forming the hologram in register over the hologram. The suggested color mask, while being a "simple concept" does not permit the formation of multicolor holograms enabling sufficient color separation, particularly between the green and blue, and does not enable inexpensive duplication thereof.
A similar embodiment is disclosed in U.S. Pat. No. 4,071,291 (Suzuki et al). In that patent, a striped aperture mask, in which the relative width of the stripes is particularly specified, is shifted in position to form a multicolor hologram. Reconstruction is effected by passing light from three separate light sources of appropriate color through separate masks similar to that used to form the hologram.
A technique for making high-fidelity, phase filters in which optical density images in a previously recorded and developed photoemulsion are converted to a relief image in a developed photopolymer layer is disclosed in U.S. Pat. No. 3,777,633 (Kirk). That patent does not suggest the use of such a technique for providing holograms capable of reconstructing multicolor images.